The present invention is directed to achieving a hemispheric isonification pattern from a wide-band omnidirectional hydrophone mounted on a sound-absorbing baffle. The invention obtains hemispheric directional response and uniform sensitivity over a broad band of frequencies from a hydrophone designed to exhibit a spherical directional response.
In the field of underwater acoustics, it is desirable to have a hydrophone that exhibits a hemispheric directional response. Hemispheric directional response results from a hydrophone that detects only those sound waves emanating from points within a 180.degree. sector defined from the receiving element of the hydrophone. Additionally, it is desirable for such a hydrophone to exhibit a uniform open circuit receiving response over a wide frequency band; that is, the hemispheric response pattern should be relatively uniform over a broad range of frequencies. Such a device is particularly useful for determining the distance between an array of such hydrophones and an underwater object. Devices of this type might typically be mounted in some fashion on a submarine, and thus must also be designed to withstand pressures exerted at submarine depths.
Certain commercially-available hydrophones are particularly well-suited to this type of application, possessing an optimum combination of several competing performance parameters including depth capacity, charge sensitivity and diffraction field size. One such hydrophone consists of a thin-walled spherical shell of piezoelectric ceramic waterproofed by a thin shell of rubber or polyurethane encapsulant. This type of hydrophone, however, normally exhibits a spherical isonification pattern, detecting sounds emanating from points within a 360.degree. arc defined from its receiving element.
Previous efforts to achieve hemispheric isonification of a spherical hydrophone while maintaining desired performance characteristics have met with limited success, and typically have involved mounting the hydrophone on a sound-reflecting surface. While the reflecting surface blocks sound waves emanating from points behind the hydrophone, thus achieving a substantially hemispheric isonification pattern, a significant problem is presented by the sensing of standing waves in the vicinity of the hydrophone generated by sound impinging on the face of the reflecting surface nearest the hydrophone. This problem is greatest at higher frequencies, where short wave reflections tend to destroy the uniformity of the hemispherical pattern. Prior approaches to solving this problem involved the use of multiple hydrophones, each of which exhibit satisfactory response characteristics over a limited frequency range. To achieve broad band response with limited interference from standing waves, it was necessary to switch from one hydrophone in the array to another hydrophone as the frequency varied.
The present invention solves the standing wave problem by mounting an omnidirectional hydrophone adjacent to an acoustic baffle, resulting in hemispheric directional response and uniform frequency response over a broad range of frequencies. The present invention offers the additional advantages of simplicity of design and fabrication.